Yes! The DoD uses the military to test novel vaccines with service members. That is part of the “some workers” because you’re being deployed worldwide to new situations.

Historically, though, I believe the DoD started it because of the threat of biological/chemical warfare, i.e. https://en.wikipedia.org/wiki/Anthrax_Vaccine_Immunization_P...

From 2001:

  In Court, it was ruled that vaccination could not be forced on military personnel without a special order by the president. Thereafter it ran into and judicial obstacles (mainly concerning the methods and viability of the vaccine).